Electrolytes are frequently used in an (aqueous) solution state. However, the aqueous solution state is recently replaced with a solid state because the solid state has easy processing characteristics in the case of applying it to electric and electronic materials and there are recent tendencies of lightweight, thin, short and small size, and saving electric power.
Conventionally, both of inorganic compounds and organic compounds are known as a proton conductive material. An example of the inorganic compounds is uranyl phosphate, which is a hydrate. These inorganic compounds have insufficient contact in the interface and have many problems in forming a conductive membrane on a substrate or electrode.
On the other hand, examples of the organic compounds are polymers belonging to cation exchange resins, for example, sulfonated vinyl polymers such as polystyrene sulfonic acid, perfluoroalkyl sulfonic acid polymer represented by Nafion (Trade name, Du Pont Co., Ltd.), perfluoroalkyl carboxylic acid polymer, and polymers prepared by introducing a sulfonic acid group or phosphoric acid group into a heat resistant polymer such as polybenzimidazole or polyether ether ketone (Polymer Preprints, Japan, Vol. 42, No. 7, pp. 2490–2492 (1993), Polymer Preprints, Japan, Vol. 43, No. 3, pp. 735–736 (1994), Polymer Preprints, Japan, Vol. 42, No. 3, p. 730(1993)).
Sulfonated vinyl polymers such as polystyrene sulfonic acid, etc, however, have a problem of inferior chemical stability (durability). A perfluorosulfonic acid electrolyte membrane is difficult to be produced and very expensive. On this account, it has difficulties in application for general use, such as automobile and household fuel cells, etc and is applicable for specially limited uses. After the use thereof, the perfluorosulfonic acid electrolyte membrane, further, has a great environmental problem in waste treatment because it has a large amount of fluorine atom in its molecules. Polymers prepared by introducing a sulfonic acid group or phosphoric acid into a heat resistant polymer such as polybenzimidazole, polyether ether ketone etc also have a problem of inferior resistance to hot water and durability.
On the other hand, sulfonated aromatic polymers are known as a proton conductive material which is industrially produced in low cost and has excellent resistance to hot water and durability. The sulfonated aromatic polymers are usually prepared by polymerizing an aromatic compound to prepare a polymer and then allowing the polymer to react with a sulfonating agent to introduce a sulfonic acid group into the polymer.
However, conventional methods have many problems such that the production risk is high because of using a large amount of the sulfonating agent such as concentrated sulfuric acid, fuming sulfuric acid, chlorosulfuric acid etc in introducing sulfonic acid, and further plant materials have limitation and the load of waste fluid treatment is high in recovering the polymer. The conventional methods, further, have problems of no facility of controlling the amount and the introducing position of the sulfonic acid group introduced into the polymer.